Quicksilver parts are manufactured by Mercury Marine and are from the P&A (parts and accessories) side of the company. Some parts, for example oil filters are available as either Mercury or Quicksilver. Same part, different packaging.
Where there is a choice Quicksilver parts are 100% the same as the equivalent Mercury part except for the labeling or packaging. In some cases replacement parts are only packaged as Quicksilver.
The difference is that some Quicksilver parts can be purchased from non-Mercury dealers in addition to Authorized Mercury dealers.
Mercury labeled parts are only available from Authorized Mercury dealers.